1. Field of the Invention
The present invention pertains to a slip assembly for gripping pipe and other tubular goods. More particularly, the present invention pertains to a slip assembly for gripping heavy weight pipe and other tubular goods, while reducing or eliminating damage to exterior surfaces of such pipe and/or tubular goods.
2. Brief Description of the Prior Art
Conventional drilling rigs typically comprise a supportive rig floor, a substantially vertical derrick extending above said rig floor, and a traveling block or other hoisting mechanism which can be raised and lowered within said derrick. In addition to other tasks, such equipment can be used to insert and remove pipe and other tubular goods from a wellbore situated below said rig floor and derrick.
During pipe installation operations, pipe is typically inserted into a well in a number of different sections of roughly equal length. These sections, often called “joints,” are typically screwed together or otherwise joined end-to-end at a drilling rig in order to form a roughly continuous “string” of pipe that extends into said wellbore. As the bottom end of the pipe string penetrates deeper into a wellbore, additional joints of pipe are added to the ever lengthening pipe string at the drilling rig. Conversely, when a pipe string is being removed from a wellbore, the upper portion of said pipe string is pulled from the well and one or more joints are unscrewed in the rig derrick until a desired length of pipe has been retrieved from said well.
The process of inserting a string of pipe in a well is typically commenced by lowering a first section of pipe into a wellbore at the rig floor, and suspending said section of pipe in place using a set of “lower slips.” When set, such lower slips—also sometimes referred to as “spider slips”—generally surround an opening in the rig floor (which is aligned with the upper opening of said well) through which the upper end of said first section of pipe protrudes. The lower slips grip against the external surface of said pipe section and hold such pipe in place.
Thereafter, a second section of pipe is lifted within the drilling rig derrick and suspended in said derrick from the rig's traveling block, top drive unit or other hoisting mechanism. Said second section of pipe is then positioned in axial alignment above the first section of pipe which was previously run into the well and suspended from the lower slips. The lower end of said second pipe section is then connected to the upper end of said first pipe section.
Once said pipe sections are joined, the pipe string can be suspended from the rig's traveling block (or top drive unit or other hoisting mechanism), allowing the total weight of said pipe string to be suspended within the derrick. In the position, the lower slips can be disengaged from gripping the pipe. The joined sections of pipe can then be lowered further into the wellbore using the rig's traveling block, top drive unit or other hoisting mechanism. After the pipe string has been lowered a desired length within the wellbore, said lower slips can again be engaged to grip the pipe string and hang it in place within said wellbore. Thereafter, the process can be repeated until a desired length of pipe has been inserted into the well.
Conventional slip assemblies (including, without limitation, lower or “spider” slip assemblies) typically utilize a plurality of wedge-shaped slip members, movably disposed within a bowl-like structure, in order to selectively grip against the outer surface of pipe and/or other tubular members. Each such slip member typically has a substantially vertical surface and a tapered back side. Although the angle of such taper can vary, conventional slip members typically have a taper angle of 9.46 degrees. The substantially vertical surface has gripping teeth or “dies” and is oriented inwardly toward the pipe or other tubular member, while the tapered back side is oriented facing away from such pipe or other tubular member and is slidably mounted in said slip bowl. The inwardly facing gripping surface can also have an arcuate shape that conforms to the outer surface of a section of pipe or other tubular to be gripped.
When pipe is gripped by a slip assembly, the tapered outer surfaces of the slip members, in cooperation with the corresponding tapered inner surface of a slip bowl in which such slip members are received, force said slip members radially inward around the outer surface of the pipe. Such slip members essentially wedge between the slip bowl and the pipe, thereby causing said slip members to grip against the outer surface of the pipe. As a result, the greater the axial load acting on said slip members, the greater the radial gripping force that said slip members impart on said pipe.
Designing of slip assemblies in general, and slip members in particular, can be very challenging. Generally, the greater the taper angle of such slip members, the lower the radial force that said slip members impart on pipe. Conversely, the lower the taper angle of such slip members, the greater the radial force that said slip members generally impart on pipe. Thus, if a slip taper angle is too large, the slip members will not create enough radial force to grip against the outer surface of the pipe. However, if a slip taper angle is too small, such slip members may impart too much radial force on pipe, thereby damaging the outer surface of such pipe and, in extreme cases, causing such pipe to collapse or crush.
Slip assembly design is further complicated by changes in pipe and other tubular goods used in modern wells. As wells are being drilled to deeper depths and/or in more challenging environments, pipe and other tubular goods used to service and equip such wells is evolving. For example, casing used in such wells is often larger and/or heavier than casing used in other wells. Further, casing, production tubing and/or other tubular goods are often constructed of and/or coated with premium materials that are less rugged and more easily damaged than conventional tubular materials. Care must be taken that slip members do not damage such pipe and/or other tubular goods including, without limitation, as a result of excessive radial gripping forces.
Conventional slip assemblies have proven to suffer from some significant limitations. Under extreme loading conditions, conventional slip assemblies can cause a crushing effect that can damage pipe and other tubular goods gripped by such slip assemblies. This crushing effect can be lessened by increasing the taper angle of slip members and a cooperating slip bowl. However, when this angle is increased, the slip members may have difficulty engaging against and gripping a section of pipe, especially when such slip members must engage against a relatively light load (such as, for example, when relatively few pipe sections are joined to a pipe string).
Thus, there is a need for a slip assembly that can be used to grip and convey heavy weight pipe and/or other tubular goods, such as casing, during installation and retrieval operations. Such slip assembly should be capable of generating sufficient radial force to securely grip heavy weight pipe and other tubular goods, while preventing crushing and/or damage to the external surfaces of such pipe and tubular goods.